This invention relates to an electric motor unit, and more specifically, to an electric motor unit for rotating a polygonal mirror.
In general, an optical deflector of a polygonal mirror type is provided with a motor unit to rotate a polygonal mirror at high speed, e.g., scores of thousands of revolutions per minute. In such a motor unit, friction between a motor shaft and bearings should be minimized for high-speed rotation. To meet this requirement, Japanese Patent Publication No. 6854/78 teaches a motor unit of a tilting pad type. In the motor unit of the tilting pad type, the motor shaft is radially supported by journal bearings of a dynamic pressure type, and is suspended so as to be axially supported by a repulsive force produced between permanent magnets individually fixed to an end of the motor shaft and a motor housing. Having its motor shaft supported both radially and axially, this motor unit is suitable for high-speed rotation, although it has the certain drawbacks. Suspended by the repulsive force between the pair of permanent magnets, the motor shaft is liable to vibrate due to external vibrations or the like, and the rotation may become somewhat eccentric. Moreover, the arrangement of the permanent magnets along the axis of the motor shaft increases the size of the motor unit.
The use of a motor unit having these problems in the optical deflector of the polygonal mirror type will cause the following additional problems. Since the incidence position of a laser beam incident upon the polygonal mirror changes as the motor shaft vibrates along its axis, the polygonal mirror must have a sufficient width. Therefore, the polygonal mirror increases in cost and weight, so that the rotatory load on the motor unit increases, which reduces the starting capability of the motor unit. In a deflector so designed that the light reflecting surfaces of the polygonal mirror are at an angle to the axis of the motor shaft, the scanning rate of a laser scanned by the polygonal mirror varies as the incidence position of the laser beam is changed by the vibration of the motor shaft.
U.S. patent application Ser. No. 411,959 filed on Aug. 26, 1982, now U.S. Pat. No. 4,443,043, a motor unit intended to solve the above noted problems inherent in the conventional motor unit. In this U.S. Application, the motor shaft is supported by a pair of radial bearings of a dynamic pressure type and by the attractive force between pairs of ring magnets. According to the magnetic thrust bearing employed in the motor unit, the motor shaft is floated by the attractive force. Thus, the motor shaft is unlikely to vibrate even if external force is applied thereto. In addition, since the shaft and the ring magnets are coaxially arranged, enlargement of the motor unit can be prevented. However, the motor unit proposed in the U.S. Application necessitates high assembly precision, rendering it necessary to allow sufficient time for processing, assembling and adjustment. Naturally, production of the motor unit in question is low in productivity, making it difficult to keep down the manufacturing cost of the motor unit.
To be more specific, one of the paired radial bearings of dynamic pressure type is fixed to the motor housing, with the other radial bearing being fixed to a cover detachably fixed to the motor housing, in order to facilitate the assembly and disassembly of the motor unit. Thus in the assembly of the motor unit, the motor shaft and the paired radial bearings must be aligned coaxially within a margin of error of about 5 .mu.m. It follows that it is likewise necessary to make the cover, housing, bearings and shaft with sufficiently high accuracy. In addition, it is necessary to allow sufficient time for alignment, leading to low productivity and a high manufacturing cost.